A typical dispatch two-way radio communication system comprises communication units, communication resources, communication sites, and a communication resource allocator. Each of the sites have a substantially distinct coverage area and are geographically located throughout the system. Each site also has a number of communication resources assigned to it, where at least one of the communication resources is used as a control channel, while a number of the remaining communication resources are used as voice channels. Such systems are known to use both frequency division multiplex access (FDMA) and time division multiple access (TDMA) methods to rebroadcast transmissions.
The communication units are typically arranged into communication groups (talkgroups) and may be located anywhere within the system (in any site). When a communication unit of a talkgroup requests a group call, it transmits, via a control channel of the site in which it is located, an inbound signaling message to the communication resource allocator. (A group call typically allows all members of the same talkgroup that are located within the system to communicate with each other.) The inbound signaling message generally comprises the requesting communication unit's individual identification number, the requesting communication unit's talkgroup, and a request for a group call. Upon receiving the inbound signaling message, the communication resource allocator will allocate a voice channel in each site to the requesting communication unit's talkgroup.
Thus, in FDMA and TDMA dispatch systems a forward link is established (one in each site where communication units are present) and monitored by all units involved in the group call in that site, and a single reverse link which is used by the group member who is currently transmitting to the other members. Non-transmitting talkgroup members are typically in a listen only mode (i.e., not able to transmit when another member is talking) and thus are not allocated a dedicated reverse link. In a TDMA system, for example, time slots are allocated to different users. A talkgroup member transmitting on an assigned reverse link, is allowed to use full power transmission to support short bursts of signals in allocated time slots. Despite the discontinuous or discrete nature of this approach, the listener receives what appears a continuous service.
In the last decade, in response to an ever-accelerating worldwide demand for mobile and personal portable communications, spread spectrum digital technology, of which one type is known as CDMA, has achieved much higher bandwidth efficiency for a given wireless spectrum allocation, and hence has proved to be an excellent alternative for serving large populations of multiple access users, than analog or other digital technologies. CDMA relies on processing power to extract a coded signal embedded across a broad frequency spectrum. The only way to extract the wanted signal from among many other overlaid unwanted signals is to have the right code. The use of coding allows more channels to be derived by the overlaying of carriers one over another and greatly enhances performance in terms of derived channels per hertz of bandwidth.
CDMA is well suited for cellular communications, but has never been employed in a conventional dispatch system. Current CDMA systems employ a form of forward power control. This means that, in a typical one-to-one (cellular) conversation, the communication unit periodically informs the base station how well it is receiving the outbound signal. If possible, the base station reduces its outbound power. If necessary, the base station increases its outbound power. This communication between the base station and the communication unit requires a two-way communication link between the two. In a dispatch situation, the most intuitive outbound power control scheme is for the base station to respond to requests to increase the power coming from any unit which requires it. However, this can only occur if all units involved in the dispatch call have an established two-way communication link.
CDMA systems also use soft hand-off at cell (site) boundaries which, in a dispatch setting, would require each of the listening units to signal the base station when another cell is found to have sufficient (usually greater) signal strength to handle the call as the communication unit migrates away from the base station, and closer to another base station. In turn, the base station will enable the now closer base station to both send and receive the same traffic to and from the migrating communication unit. In order for this procedure to take place, the communication unit must have a communication path in to the fixed end.
Reverse power control is another important aspect of CDMA systems. (In fact, this is much more important than forward power control.) Reverse power control attempts to equalize the received signal powers for all communication units controlled by a particular base station. By having the base station monitoring (and accordingly vary) power from listening communication units, the efficiency of CDMA is realized.
In order to avoid excessive interference on the reverse link, communication units accessing the channel for the first time (i.e., before reverse power control has been established) use what is known as "access probes". This means that they access the channel with low power, and slowly ramp up their power until a response from the fixed end tells them that their signal has been received. For cellular communications, the time delay caused by this access method is insignificant. On the other hand, the time delay in setting up a link for a follow-up call during a dispatch group call would be significant and must necessarily have been established in advance. In dispatch, any one of the talkgroup members may want to send an inbound message and then drop back to listening status. If each access required access probes, the delay would be intolerable for a quality dispatch service.
As stated above, non-CDMA systems do not have multiple reverse link connections in a dispatch call because the nature of the access methodology (TDMA or FDMA) does not demand it (quality dispatch functionality is possible for listening units without the power control and soft hand-off required in CDMA systems). TDMA and FDMA systems assemble a talkgroup by alerting the members of the call, and informing them of where to tune to listen to the call. The establishment of multiple reverse (inbound) signaling links is simply not an issue. For CDMA systems, there are to date only point-to-point communication protocols established (i.e., dispatch capability has never been deployed). Therefore, the establishment of multiple reverse signaling links and the conflict that this can cause with the strict timing requirements of a dispatch service have not been addressed.
A hybrid TDMA/CDMA system indirectly addressing the delay problem with CDMA channel access was proposed in U.S. Pat. No. 5,295,152, entitled "TDMA for mobile access in a CDMA system". In the proposed system, a separate TDMA channel is defined, either separated in frequency band or time interleaved in the same frequency band as the CDMA channel. Because TDMA does not require stringent inbound power control, immediate access on the TDMA channel portion is possible without the access probe delays seen in CDMA. This TDMA channel could therefore be used for access and control signaling (e.g., fast follow-up in a dispatch call), while the CDMA portion of the system would be used for the traffic channels. Such a hybrid approach, however, would require essentially two different types of radio equipment (TDMA and CDMA) at the base and mobiles ends, leading to higher costs, and may present interference problems to nearby, pure CDMA systems.
Providing all listening-only units on a CDMA system with a full traffic capable reverse link would quickly deteriorate system performance in a heavy user system due to excessive interference and would be a great waste of resources. CDMA systems are interference limited, in that capacity is determined by the amount of interference due to all the users on the system. An alternative might be to set up non-talking group members in a listen-only mode with no reverse link whatsoever. However, this latter scenario would lead to serious difficulties with forward power control, soft hand-offs, and rapid channel access for the follow-on calls in the dispatch conversation (group call).
It would be a great advancement in the art of dispatch communications to be able to provide quality dispatch service on a CDMA based wireless system.
Reverse links are necessary in a CDMA dispatch system since such links provide to listening group call members the means for inbound power control to enable fast access in response to the dispatch call, the means for outbound power control, and the means for enabling soft hand-off. The establishment of these links, however, could seriously impact the call setup time, and thus service quality. There is also a further need therefore for a CDMA based wireless system which establishes reverse links for all talkgroup members participating in a dispatch call but that does so without seriously negatively impacting the call set up time and thus service quality.